The present invention relates to a method for implementing a logic operation in quantum computing devices constructed by Josephson coupled systems.
Quantum computers are new computers actively using the basic principle of quantum mechanics. The quantum computer can solve a specific problem, for example, the prime factorization of large natural numbers, much faster than conventional classical computers can do. The quantum computer uses a quantum two-level system, called a quantum bit, corresponding to a bit used in the classical computer.
In other words, the basic unit of the information in the quantum computer is defined by a quantum bit (qubit). A qubit corresponds to, for example, an atom (ion) having different quantum states. Two of the quantum states are used to store digital information.
There are candidates for quantum two-level systems. From the viewpoint of quantum bit integration, solid-state devices are promising. A quantum bit using a superconducting device has good coherence. Accordingly, the quantum bit using the superconducting device holds a big lead in the solid-state devices.
Japanese Unexamined Patent Application Publication No. 2000-277723 (herein below, referred to as Patent Document 1) discloses a quantum computing device whereby it is unnecessary to extract high-speed signals, thus easily reading out the result of a computation. In the quantum computing device, a quantum bit is formed by a quantum box electrode and a counter electrode, or formed by a superconducting box electrode and a superconducting counter electrode. The quantum bit is controlled by a gate voltage that is applied to a gate electrode. A probe electrode is coupled to the quantum bit via a tunnel barrier. The probe electrode can read out the state of the quantum bit after computation and also can prepare the initial state thereof before computation. An electrostatic potential of the superconducting box electrode is controlled by the gate voltage applied to the gate electrode. Thus, a transition In Cooper-pair tunneling through the tunnel barrier, namely, the state of the quantum bit is controlled. The probe electrode is biased to a positive voltage. So long as at least one excess Cooper pair exists in the superconducting box electrode, the probe electrode extracts two electrons with two quasiparticle tunneling events through the tunnel barrier to observe the state of the quantum bit.
The Cooper pair will now be described. In normal metals, a weak Coulomb repulsion acts between electrons. Electrons move independently of each other. On the other hand, when an attractive interaction acts between electrons even slightly, an energy produced by a pair of electrons, of which momentums are equal in size and opposite in direction, is lower than that produced by electrons moving independently of each other. The pair of electrons is called a Cooper pair. In metals in each of which the attractive interaction acts between electrons, when energy saved by generating Cooper pairs is higher than that of thermal agitation, many electrons are paired, thus condensing into one energy state. This state corresponds to superconductivity. The phenomenon of perfect diamagnetism (Meissner effect) is explained based on the fact that the condensed Cooper pairs have the same phase and all of the Cooper pairs can be described by one wave function.
A quasiparticle will now be described. In a superconducting metal, many electrons are paired as Cooper pairs and are condensed into one energy state. When energy (superconducting gap energy) of a predetermined level or more caused by lattice vibration or external light irradiation is applied to the Cooper pairs, each Cooper pair is broken into two electrons. The electrons are in a superconductor excited state. The state of each of the two electrons is different from that of a free electron in a normal metal. Therefore, the electron In this state is called a quasiparticle in order to distinguish from the normal free electron. In a tunnel function including two superconducting electrodes, a quasiparticle current steeply increases by a voltage corresponding to the sum of gap energies of both the superconducting electrodes. Thus, the current-voltage characteristic exhibits strong non-linearity.
One-bit operation of the quantum bit using superconducting devices has been reported In “Nature (ENGLAND)”, Vol. 398, pp. 786–788, Apr. 29, 1999. It is known that when one bit gate for controlling one bit is combined with a two-bit gate, called a controlled-NOT gate, all of operations necessary for quantum computing is made possible.
Therefore, realizing a controlled-NOT gate in quantum bit using superconducting devices is of extreme importance.
As shown in FIG. 1, a controlled-NOT gate has an input composed of a control bit and a target bit, namely, two quantum bits. Only when the state of the control bit is “0”, the value of the target bit is inversed. Although this definition is different from the general one, where the value of the target bit is inversed only when the state of the control bit is “1”, this difference gives no essential restriction.
A theoretical approach to realize a controlled-NOT gate using superconducting charge quantum bits has been reported in “Physical Review Letters”, Vol. 79, pp. 2371–2374, Sep. 22, 1997. However, the controlled-NOT gate requires a large inductance to couple two quantum bits. Disadvantageously, it is difficult to realize this controlled-NOT gate,
One approach to coupling quantum bits uses capacitance. The fabrication of devices for this approach is easier than that using inductance. Furthermore, the devices for this approach can be made compact. Actually, superconducting charge quantum bits coupled by using the capacitance have already been produced. The quantum oscillation of the quantum bits has been observed (“Nature (ENGLAND)”, Vol. 421, pp. 823–826, Feb. 20, 2003).
However, any method for producing a controlled-NOT gate in superconducting charge quantum bits coupled by using the capacitance has not been proposed.